This invention relates to a filtration apparatus utilizing multiple spiral wound filtration cartridges.
In filtration, it is desirable to maximize filter area while minimizing the volume of the filter construction employed. One common means for attaining this result is to provide a construction formed by spirally winding a multi-layer material having a filter layer to form a tightly wound cylinder into which liquid is introduced at one spiral end and removed from the opposite spiral end. Means must be provided in the filter construction for assuring that all entering liquid passes through the filter medium prior to being removed, that is, the entering liquid must be prevented from simply passing through the spaces between the wound filter material without passing through the filter medium. Such constructions are shown, for example in U.S. Pat. No. 3,962,097.
A problem encountered in spiral wound construction results from the fragile characteristic of some membranes, such as ultrafiltration membranes which tear during filter cartridge construction.
In one process, spiral wound filters are formed of multiple layers of membrane filter and screens wherein a membrane filter layer is folded onto itself to form a V shape and two filter layers. Multiple leaves of membrane/screen stacks are glued in a flat orientation, and then rolled onto a perforated core. Layers are formed of feed and permeate screens. During the rolling operation, to form the spiral wound filter, because the membrane layers are sticky, they tend to wrinkle instead of sliding past each other. As a result, retention integrity is compromised. Flow properties are good, as negligible parasitic losses are measured from flow through the relatively short permeate channel (about 18 inches long).
In a second process, a single long leaf is used to form the spiral device. Glue is applied only in the rolled form. This limits the number of leaves to one, but reduces the stress on the membrane as it is being rolled. In a single layer format, devices made with this technique are shown to be integral. However, because the permeate channel is long (about 40 inches) the parasitic losses, primarily due to high pressure drop, in that channel are unacceptable high.
Accordingly, it would be desirable to provide a filtration apparatus which utilizes spiral wound filter cartridge construction to minimize permeate flow losses while providing satisfactory permeate production capacity.